1. Field of the Invention
The present invention relates to seismic prospecting. More particularly, it relates to a system for reducing the effects of a monochromatic interference signal on seismic data records.
2. Description of the Prior Art
Certain monochromatic interference signals have always been a problem in seismic prospecting. In the United States, the most prevalent signal of this type is caused by 60 cycles per second power distribution systems. In the past, this interference signal has been removed from seismic records by various filtering techniques including filters incorporated into field systems and computer programs that remove the interference signal during data processing.
A disadvantage of previously used filtering techniques is that either additional hardware must be incorporated into the seismic field equipment or additional computer programming must be used during seismic data processing.
Another disadvantage of most filters is that they cause distortion in seismic data signals at frequencies near the interference signal frequency. The filters that have been developed for removing a monochromatic interference signal without distorting the data signal at nearby frequencies subtract from the data signal a fixed nulling signal. The nulling signal frequency, phase, and amplitude characteristics are adjusted to match the interference signal, either manually (see U.S. Pat. No. 2,733,412, Alexander et al, issued Jan. 31, 1956) or by feedback loops (see U.S. Pat. Nos. 3,757,235, McCormack et al, issued Sept. 4, 1973, and 3,889,229, Kostelnicek et al, issued June 10, 1975). Both the manually adjusted systems and the feedback systems, however, require the use of additional complex hardware, and the manually adjusted systems are time consuming for the field personnel to operate. Furthermore, to avoid distorting nearby frequencies, the nulling signal must not be varied during the recording of the seismic signal. And if the interference signal characteristics change slightly after the nulling signal is fixed, cancellation of the interference signal may be inadequate.